1. Field of the Invention
This invention relates to a method for recovery of polyarylene sulfides from a reaction mixture after synthesis, and in particular, to a method for refining polyarylene sulfides as synthesized.
Because of their excellent heat resistance and chemical resistance properties, polyarylene sulfides have drawn a good deal of attention as materials for preparing electrical and electronic parts, automotive parts and the like.
Polyarylene sulfides are moldable or shapable into various articles such as parts, films, fibers, etc. by means of, for example, injection molding and extrusion molding techniques and have been employed in a variety of fields where their heat resistance and chemical resistance properties are desired or favored.
2. Prior Art
One of the typical processes for producing polyarylene sulfides is disclosed in Japanese Patent Publication (KOKOKU) No. 45-3368, which comprises reacting a dihalo-aromatic compound with an alkali metal sulfide, such as sodium sulfide, in an organic amide solvent, such as N-methyl pyrrolidone.
Since there is a general tendency that the polyarylene sulfides produced by such a process have relatively low molecular weights resulting in relatively poor mechanical properties, it have been proposed to prepare polyarylene sulfides having increased molecular weights by means of modified polymerization processes, for example in Japanese Patent Publications (KOKOKUs) Nos. 52-12240 and 57-334 and Japanese Patent Public Disclosures (KOKAIs) Nos. 61-7332 and 61-66720. However, a polyarylene sulfide product synthesized by such a modified process contains not only molecules of increased molecular weights but also molecules of little or not increased molecular weights. The presence of the lower molecular weight fraction appears to arrest development of improved mechanical properties which, otherwise, could be achieved by the higher molecular weight fraction.
Japanese Patent Public Disclosure (KOKAI) No. 59-1536 discloses a typical method for refining polyarylene sulfide in which water or the like-is added to the polymerization system so as to recover the polyarylene sulfide in the form of particultes. Again the thus recovered polyarylene sulfide is not sufficiently freed from the low molecular weight molecules to provide satisfactory mechanical properties.
Japanese Patent Public Disclosure (KOKAI) No.62-232437 discloses a method in which a polymerized slurry containing a polyarylene sulfide is filtered at temperatures in the range of from 50.degree. C. to the boiling point of the slurry. It is difficult to filter the polymerized slurry as such. Therefore, in practical application, it is necessary to dilute the polymerized slurry with a large amount of expensive solvent that is the sane as the solvent used in the preceding polymerization stage. Employment of such a high degree of dilution will disadvantageously lower the, productivity of process. Even if an undiluted or neat slurry from the preceding polymerization stage can be filtered, this will not provide a satisfactorily refined polyarylene sulfide.